I had bookmarked this “simple regex language” for regex a while ago, but this morning as I revisited it I was struck by just how neat this project is:
begin with any of (digit, letter, one of "._%+-") once or more,
literally "@",
any of (digit, letter, one of ".-") once or more,
literally ".",
letter at least 2 times,
must end, case insensitive
…which produces this regular expression:
/^(?:[0-9]|[a-z]|[\._%\+-])+(?:@)(?:[0-9]|[a-z]|[\.-])+(?:\.)[a-z]{2,}$/i
You can see the project at https://simple-regex.com or check it out at GitHub.